A reducing agent supply apparatus for supplying reducing agent to an exhaust gas purification catalyst provided in the exhaust system of an internal combustion engine has previously been known. Some reducing agent supply apparatuses have a reducing agent addition valve provided upstream of the exhaust gas purification catalyst in the exhaust system of the internal combustion engine. In this case, reducing agent can be supplied to the exhaust gas purification catalyst by injecting liquid reducing agent into the exhaust gas through the reducing agent addition valve.
Patent document 1 discloses an apparatus that diagnoses the above-described type of reducing agent supply apparatus having a reducing agent addition valve. The diagnosis apparatus according to patent document 1 has a temperature sensing unit that senses the temperature of the tip end portion of the reducing agent addition valve. The apparatus determines whether or not there is a malfunction in the reducing agent supply apparatus based on the temperature decrease of the tip end portion of the reducing agent addition valve as the reducing agent is supplied.
In this connection, the temperature decrease of the tip end portion of the reducing agent addition valve sensed by the temperature sensing unit in the diagnosis apparatus according to patent document 1 is caused by cooling of the tip end portion of the reducing agent addition valve, which is exposed to high temperature exhaust gas, by the reducing agent. Therefore, if the quantity of reducing agent injected through the reducing agent addition valve or the quantity of reducing agent passing by the tip end of the reducing agent addition valve decreases due to the occurrence of a malfunction, such as clogging of the reducing agent addition valve, in the reducing agent supply apparatus, the temperature decrease of the tip end portion will become smaller.
However, even when the reducing agent supply apparatus is in normal condition and the quantity of reducing agent passing by the tip end of the reducing agent addition valve is the same, a change in the temperature of the reducing agent itself may lead to a change in the temperature decrease of the tip end portion of the reducing agent addition valve during the supply of the reducing agent. For this reason, it is sometimes difficult to properly determine whether or not there is a malfunction in the reducing agent supply apparatus based on the temperature decrease of the tip end portion. In addition, mounting the temperature sensing unit on the tip end portion of the reducing agent addition valve may sometimes involve difficulties in achieving reliability in the reducing agent addition valve or due to its influences on the reducing agent injection characteristics.
Furthermore, in the case where the exhaust gas purification catalyst provided downstream of the reducing agent addition valve in the exhaust system of an internal combustion engine is a catalyst having an oxidizing ability, there is a known method of estimating the quantity of reducing agent added through the reducing agent addition valve based on the temperature rise of the exhaust gas purification catalyst caused as the reducing agent added through the reducing agent addition valve is oxidized in the exhaust gas purification catalyst. Whether or not there is a malfunction in the reducing agent supply apparatus can be determined based on the quantity of added reducing agent estimated by this method.
However, in some cases, no heat of the oxidation of reducing agent is generated in the exhaust gas purification catalyst, as is the case where the exhaust gas purification catalyst is an NOx selective reduction catalyst and urea is added as the reducing agent through the reducing agent addition valve. In such cases, it is difficult to determine whether or not there is a malfunction in the reducing agent supply apparatus by the above-described method.